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OFDM Transceiver Design for Optimizing
Sensitivity and Long-Haul Performance
Jochen Leibrich, Member, IEEE , Abdulamir Ali, and Werner Rosenkranz, Member, IEEE
Abstract— Design parameters for optical OFDM transceivers
based on intensity modulation and direct detection are
considered. Proper selection of modulator bias improves
sensitivity by several dB. The length of the guard interval is
adapted for uncompensated long-haul transmission.
Index Terms—OFDM, sensitivity, transceiver technology.
I. OFDM IN OPTICS - A BRIEF OVERVIEW
II. OOFDM BASED ON DIRECT DETECTION
A. Back-to-Back System Setup
Fig. 1 depicts the system setup. The binary data stream at
43Gb/s is parallelized and mapped into complex symbols. By
means of the choice of symbol mapping (e.g. QPSK, QAM),
bandwidth efficiency and sensitivity can be balanced. In this
contribution we use QPSK mapping onto 512 subchannels.
The complex symbols are fed into an IFFT. To generate a
real output signal, complex conjugate symbols are created
appropriately and fed into the IFFT, too. Moreover, half of the
input of the IFFT around the DC value is filled with zeros to
create an OFDM spectrum that avoids interference with
second-order nonlinearity after squaring in the photo diode
[1]. Alternatively, this signal can be constructed by means of a
complex IFFT followed by electrical I-Q up-conversion. After
serialization, the guard interval may be added resulting in the
electrical quasi-analog signal to be transmitted over the optical
The authors are with the Faculty of Engineering, University of Kiel,
Kaiserstrasse 2, 24143 Kiel, Germany (e-mail:[email protected]).
Data
IFFT
Input S/P
Guard
QPSK P/S
Data
Output QPSK-1
P/S
SSB
Filter
MZM
OFDM modulator
OFDM demodulator
OFDM
Equalizer
RTHOGONAL frequency division multiplexing (OFDM) is
known in classical digital communications. Optical
communications, however, is currently accelerating its effort
to implement sophisticated transmitter and receiver structures
to increase data throughput. Consequently, investigation into
optical OFDM nowadays is a very hot topic [1-4].
There are two strategies for transmitting the quasi-analog
OFDM signal. One solution is given by optical I-Qmodulation in conjunction with coherent detection
(CO-OFDM) [2,3]. A second method restricts to a real OFDM
signal transmitted with intensity modulation and direct
detection and is simply called optical OFDM (OOFDM).
While CO-OFDM shows higher performance in terms of
bandwidth efficiency and receiver sensitivity, OOFDM
provides the advantages of OFDM, like robustness towards
fiber dispersion, with low effort. Hence, here we focus on the
latter approach.
optical
channel
O
channel. The signal is clipped appropriately and fed into an
optical Mach-Zehnder modulator (MZM). After transmission,
a photodiode (PD) converts the optical signal into the
electrical domain, where OFDM demodulation and frequencydomain equalization are performed. For back-to-back
transmission, the channel is equal to the identical system. In
this case, guard interval and equalizer are not required.
S/P
Guard-1
FFT
Fig. 1. Setup for OOFDM with intensity modulation and direct detection.
B. Guard Interval and SSB Filtering for Dispersive Fiber
In case of dispersive fiber, linear distortion and crosstalk
between OFDM symbols are obtained. To compensate, a
guard interval of sufficient length (see section IV) needs to be
introduced to allow for frequency-domain equalization (i.e.
cyclic convolution) afterwards. However, due to MZM and
photodiode the system exhibits nonlinear behavior. By means
of first-order analysis of the overall system, it can be shown
that the relation between input and output signal is given by
the real part of the complex fiber transfer function, i.e.
{
}
H DSB = Re exp ( − j 2π 2 β 2 f 2 L ) = cos ( 2π 2 β 2 f 2 L ) ,
where β2 characterizes chromatic dispersion of a fiber of
length L. Depending on the product β2L the transfer function
exhibits nulls in its magnitude resulting in complete
suppression of specific OFDM subcarriers. This can be
avoided by suppressing the lower sideband of the symmetric
spectrum. Then, first order analysis results in an overall
transfer function (i.e. including SSB filter and optical fiber) of
H SSB ( f ) = exp  − j sgn ( f ) 2π 2 β 2 f 2 L  ,
where sgn( f ) denotes the sign function. HSSB( f ) does not
show nulls in its magnitude.
III. OPTIMIZING SENSITIVITY
IV. GUARD INTERVAL VERSUS TRANSMISSION LENGTH
A. Trade-off between Noise Requirements and Nonlinearity
For investigation into receiver sensitivity, SSB filtering and
fiber are neglected. Then the nonlinearities of MZM and PD
may be combined resulting in the characteristic given in fig. 2.
For high linearity, the MZM is biased at its quadrature point.
A. Fiber Impulse Response
As mentioned in section II, the purpose of the guard interval
is to suppress crosstalk between neighboring OFDM symbols.
Therefore, the length of the impulse response needs to be
known. For wireless transmission, the length is determined
uniquely by minimum and maximum path delay. For
dispersive fiber, however, the impulse response theoretically is
of infinite length. Even in a band limited scenario, there is no
clearly defined length but the impulse response decays
asymptotically down to zero [5]. Therefore, the length of the
guard interval for a given amount of dispersion is investigated
by simulation in the following.
Ubias/Uπ=0.8
quadrature point (Ubias/Uπ=0.5)
MZM input voltage
Uπ
Fig. 2. Combined nonlinear characteristic of MZM and PD. As MZM input
voltage is DC-free, the position of the vertical axis depends on the bias point.
Intensity modulation inherently requires a carrier. For fixed
bias, carrier power is fixed, too. Now, the amplitude of the
zero-mean driving signal is varied. Obviously, there is a tradeoff: For low amplitude, carrier power is much higher than the
power in the sidebands yielding low sensitivity. For high
amplitude, however, the signal suffers nonlinear distortion.
The optimum driving amplitude, quantified by its standard
deviation normalized by Uπ, is shown fig. 3a).
OSNR[dB]@BER=10-3
a)
B. Simulation Results
Fig. 4 depicts the impact of length of guard interval on
required OSNR. Clearly, the more dispersion the longer is the
required length (given as overhead added to OFDM symbol
length). Depending on the length, uncompensated
transmission over hundreds of kilometers is feasible.
Currently, the required length depending on signal bandwidth
and accumulated dispersion is subject of more intensive study.
OSNR[dB]@BER=10-3
photo diode output current
b)
30
30
28
26
26
24
22
22
18
20
0
0.2
0.4
normalized standard deviation
14
0.6 0
0.5
0.6
0.7
20
19
18
17
16
0.2
0.4
0.6
normalized standard deviation
Fig. 3. Required OSNR for BER=10-3 over standard dev. of driving voltage
a) when biasing the MZM at quadrature point; b) with different bias points.
B. Improving Sensitivity by Means of Proper Biasing
The results in fig. 3a) show the basic dilemma when MZM
is biased at the point of highest linearity: To avoid nonlinear
distortion the driving amplitude still has to be chosen low. The
power of the optical carrier wastes a high percentage of the
total power (e.g. ≈80% for σ ≈0.3 Uπ). Therefore, in our
approach a variable bias is introduced. Choosing the bias
voltage such that carrier power is reduced (i.e. on the
characteristic given in fig. 2 we move to the left) improves
sensitivity, but at the cost of worse linearity. As long as the
resulting degradation does not overcompensate for the benefit
due to reduced carrier power, receiver sensitivity increases.
Fig. 3b) shows simulation results for different values of the
bias. Sensitivity is increased, but obviously there is not a clear
optimum value for the bias. It seems that best performance is
achieved the lower the carrier power is. However, this results
in high insertion loss of the MZM. Therefore, the bias voltage
that is allowed is limited to a certain value (e.g. 0.8Uπ).
3%
6%
25%
15
140
0.8
13%
0%
200
400
600
SSMF length [km]
800
Fig. 4. Required OSNR for BER=10-3 over SSMF length for several values of
guard interval length, normalized by OFDM symbol length.
V. CONCLUSION
The impact of OOFDM transceiver design on receiver
sensitivity and long-haul performance is shown. Optimizing
the bias increases sensitivity by approx. 5dB. Adequate length
of guard interval allows for uncompensated transmission
length of several hundreds of km of SSMF.
REFERENCES
[1]
[2]
[3]
[4]
[5]
A. Lowery and J. Armstrong, “Adaptation of orthogonal frequency
division multiplexing (OFDM) to compensate impairments in optical
transmission systems,” in Proceedings of ECOC 2007, vol. 2,
pp. 121-152, 2007.
S. Jansen, I. Morita, H. Tanaka, “10x121.9-Gb/s PDM-OFDM
transmission with 2-b/s/Hz spectral efficiency over 1,000 km of SSMF,”
in Proceedings of OFC 2008, paper PDP2, 2008.
Q. Yang, Y. Ma, W. Shieh, “107 Gb/s coherent optical OFDM reception
using orthogonal band multiplexing,” in Proceedings of OFC 2008,
paper PDP7, 2008.
A. Ali, J. Leibrich, W. Rosenkranz, “Impact of nonlinearities on optical
OFDM with direct detection,” in Proceedings of ECOC 2007, vol. 5,
pp. 217-218, 2007.
J. Leibrich, Modeling and Simulation of Limiting Impairments on Next
Generation's Transparent Optical WDM Transmission Systems with
Advanced Modulation Formats, PhD Thesis, Shaker, 2007.